We study a completely-packed loop model with crossings in a three-dimensional lattice and confirm it is described by
RPn−1 sigma field theories. We use Monte Carlo simulations, with systems sizes up to
1800×1800×1800, to obtain the critical exponents for a universality class with no previously reported estimates in the literature, namely the replica-like limit
n→1 of
RPn−1 sigma field theory. Estimates of critical exponents include
ν=0.918(5) and
η=−0.091(9). We also study the scaling dimension of the 4-leg watermelon correlators, particularly for
n=1 we obtain the value
x4=1.292(8).
An Ising chain with Hamiltonian H=-J Sigma i<j epsilon ij S i S j / mod i-j mod (1+ sigma )/2 is considered where the epsilon ij are independent random variables. According to Kotliar et al. (1983) this model has a phrase transition with non-mean-field exponents when 1 / 3 < sigma <1 and mean-field exponents beyond the upper critical value of sigma = 1 / 3 . By means of an epsilon expansion about the lower critical value of sigma =1, the model is shown to be replica symmetric as epsilon to 0 and it is speculated that this result holds for 1 / 3 < sigma <1.